A known transformer driver, in this case a Class-D amplifier, 10 is shown in FIG. 1A. The Class-D amplifier makes use of a P/N totem pole H-bridge configuration 12, 14, 16, 18 to drive two load terminals 20, 22 to alternate supply voltages in an active fashion. In FIG. 1A the alternate supply voltages are ground and VDD. The voltage between the load terminals 20, 22 is applied to an inductor 24 of an LC filter by way of a transformer 26 to induce current flow in the inductor in accordance with I=(1/L)*integral(Vout−Vfilt_out), where L is the inductance of the inductor, Vout is the voltage across the secondary winding of the transformer and Vfilt_out is the voltage across the capacitor 28 of the LC filter. A resistive load 30 is present at the output of the LC filter. The induced current flow is drawn through the transformer from whichever supply, i.e. ground or VDD, is presently driving the output. Thus energy is transferred from the supply to the inductor 24 as is shown in FIG. 1B or from the inductor back to the supply as is shown in FIG. 1C depending on the direction of the load current. The components in FIGS. 1B and 1C have the same reference numerals as the corresponding components in FIG. 1A. Assuming the transistors 12, 14, 16, 18 to be performing as ideal switches and an ideal inductor, capacitor and resistor, the only energy loss is that which results from the ripple voltage present across the load resistor 30.
Where an increase in injected power is required, the load resistance 30 is reduced. However, at some point during the course of reducing the load resistance the parasitic resistances in the circuit become significant compared to the load resistance and efficiency decreases rapidly. The parasitic resistances are present as transistor on-state resistance and parasitic series resistance in the LC filter and the transformer. If the load resistance is further reduced it becomes impossible to achieve the required injected power because the parasitic resistances are too large.
It is an object of the invention to address one or more of the above issues and provide a transformer driver of acceptable efficiency and able to provide for increased injection of power between a primary winding and a secondary winding of the transformer to handle increased output power.